FIELD OF THE INVENTION
The invention relates to a gas turbine for the combustion of a fuel gas.
A gas turbine conventionally includes a compressor part, a burner part and a turbine part. The compressor part and the turbine part are usually located on a common shaft which, at the same time, drives a generator for generating electricity. Preheated fresh air is compressed in the compressor part to the pressure necessary in the burner part. The compressed air and preheated fresh air are burnt together with a fuel, such as natural gas or petroleum, for example, in the burner part. The hot burner exhaust gas is fed to the turbine part and is expanded there.
Detailed information on the structure and use of gas turbines is provided in the company publication entitled "Gasturbines and Gasturbine Power Plants" of Siemens A G, May 1994, Order Number A 96001-U 124-A 259-V 1-7600.
The combustion of the compressed and preheated fresh air together with the fuel gas also gives rise to nitrogen oxides NO.sub.x which are particularly undesirable combustion products. Those nitrogen oxides, along with sulfur dioxide, are the main cause of the environmental problem of acid rain. Consequently, as well as in view of strict statutory norms on limit values for the emission of NO.sub.x, the aim is to keep the NO.sub.x emission of a gas turbine particularly low and, at the same time, to avoid appreciably influencing the power of the gas turbine.
Thus, for example, the lowering of the flame temperature in the burner part has the effect of reducing the nitrogen oxides. In that case, steam is added to the fuel gas or to the compressed and preheated fresh air, or water is injected into the combustion space.
Such measures, which per se decrease the emission of nitrogen oxides from the gas turbine, are referred to as primary measures for the reduction of nitrogen oxides.
Accordingly, all those measures are referred to as secondary measures in which nitrogen oxides contained in the exhaust gas of a gas turbine or even fundamentally of a combustion process are decreased through the use of subsequent measures.
In that respect, the method of selective catalytic reduction (SCR) has gained acceptance throughout the world. In that method, the nitrogen oxides, together with a reducing agent, usually ammonia, are brought into contact with a catalyst and, in that process, form nitrogen and water. The use of that technology therefore necessarily entails the consumption of reducing agent. The catalytic converters, which are disposed in the exhaust-gas duct for reducing the nitrogen oxides, naturally bring about a pressure drop in the exhaust-gas duct and that pressure drop is accompanied by a power drop in the turbine. Even a power drop amounting to a few points per thousand, in the case of a gas turbine power of 150 MW, for example, and in the case of the current-purchasing price of about $0.016/kWh (and about 0.15 DM/kWh in Germany, for example) for current, has a serious effect on the result which can be achieved with such an apparatus.
Recent considerations with regard to the construction of the burner part tend towards replacing the normally used diffusion burner or swirl-stabilized premixing burner with a catalytic combustion chamber. Lower emissions of nitrogen oxides are achieved through the use of a catalytic combustion chamber than is possible with the above-mentioned burner types. The known disadvantages of the SCR method (large catalyst volumes, consumption of the reducing agent, high pressure loss) can thereby be overcome.
One disadvantage of a catalytic combustion chamber and likewise of a conventional combustion chamber is the ignition temperature which is necessary for combustion and which, if natural gas is used, is in the region of a value of about 400.degree. C. This fact too closely restricts the operating range of the combustion chamber in a gas turbine and makes it necessary to use an auxiliary burner which naturally constitutes a source of nitrogen oxide.
In that respect, it is known from U.S. Pat. No. 5,048,284 to separate part of the fuel gas from the remaining fuel gas and to guide it through a catalytic reformer. In that case, according to the reforming process, methane and water are in equilibrium with hydrogen and carbon monoxide, so that part of the methane is converted into hydrogen. In that case, the fuel gas is either methane or a higher alkane which is decomposed to form methane. The reformed fuel gas is subsequently fed to the fuel gas again, with hydrogen alone leading to a lowering of the ignition temperature. Moreover, the reaction rate of the reforming process is relatively low.